Rail vehicles are generally equipped with service brake and/or parking brake calipers provided with a service brake and/or parking brake cylinder.
This cylinder comprises a brake piston movable under the effect of the action of one or more springs and furthermore movable under the effect of a fluid under pressure.
The displacement of the brake piston under the effect of the springs leads to a braking action such as the clamping of a brake disk between two linings, or the direct pressure of a block against a vehicle wheel.
On the contrary, the displacement of the braking piston under the effect of the fluid under pressure leads to an opposite action to the braking, i.e. the unclamping of the brake disk or the withdrawal of the block; and thereby enables the brake to be let off.
More specifically, this type of brake cylinder comprises a body in which and relative to which the brake piston is movable to act on the braking linkage via a thrust rod.
The brake cylinder also comprises a cavity formed in the body and provided to receive the springs, which is configured to place the braking piston in a braking position, via for example a bearing flange cooperating both with the spring and with the braking piston.
The brake cylinder further comprises a pressure chamber delimited in particular by the braking piston and by the body, and which is connected by a first pipe to a first source of pressure agent to return the piston into a resting position in which the brake is set but not applied.
To set the brake, the pressure chamber is supplied with pressure agent, also called fluid under pressure, via the corresponding source so as to displace the braking piston in the body.
The displacement of the braking piston towards its resting position leads to the displacement of the bearing flange against the load of the spring, which becomes compressed.
The brake cylinder is furthermore provided with a backup piston which is movable relative to the cylinder body.
This backup piston is adjacent the braking piston and delimits with it the pressure chamber.
More specifically, the braking piston has a head and a rod and the backup piston is formed by a body mounted on the rod and facing the head of the braking piston.
The brake cylinder further comprises a backup pressure chamber delimited by the backup piston and by the body, and which is connected by a second pipe to a second source of pressure agent, so as to displace the backup piston to push and return the braking piston into its resting position in which the brake is set but not applied.
The brake cylinder is provided with a first seal disposed between the head of the braking piston and the body of the cylinder, with a second seal disposed between the body of the backup piston and the rod of the braking piston, with a third seal disposed between the body of the backup piston and the body of the cylinder, and with a fourth seal disposed between the rod of the braking piston and the body of the cylinder.
In case of leakage defect in particular due to the defective character of the first seal, the resetting of the brake is ensured by the supply of the backup chamber with pressure agent via the corresponding source; so as to displace the backup piston which pushes and displaces the braking piston within the body until it reaches its resting position.